Fibers derived from 5(6)-amino-2-(p-aminophenyl) benzimidazole (DAPBI), para-phenylenediamine (PPD) and terephthaloyl dichloride (TCl, also commonly referred to as terephthaloyl chloride) are known in the art. Such copolymers are the basis for a high strength fibers manufactured in Russia, for example, under the trade names Armos® and Rusar®. See, Russian Patent Application No. 2,045,586. U.S. Pat. No. 4,172,938 discloses a process for making aramid polymer that utilizes a solvent containing at least 1.5 but less than 5 weight percent CaCl2 in N-methyl-2-pyrrolidone (NMP).
Present technology does not permit producing DAPBI-containing aramid polymers in a high solids content reaction while achieving polymer having high inherent viscosity. Such a process would be desirable.